An extremely unusual case of a cystic schwannoma in the region of the inferior vermis and posterior to the fourth ventricle in a fifteen year old boy is reported. The cystic tumour caused partial obstruction to the outflow of cerebrospinal fluid from fourth ventricle and resulted in development of supratentorial hydrocephalus. On investigations, the schwannoma simulated a Dandy-Walker cyst. The boy presented with symptoms of increased intracranial pressure. On surgery, the lesion was not arising from any cranial nerve, nor was it attached to brain parenchyma, blood vessel or to the dura. The possible histogenesis of the cystic schwannoma in a rare location is discussed.

Schwannomas are benign neoplasms of the peripheral nerve sheath and are believed to have their origin in embryonic neural crest cells. They account for approximately 8% of all brain tumours and 25% of all spinal tumours.[1] Intraparenchymal schwannomas of the central nervous system have also been rarely reported.[2],[3] We could locate one report of a intra-fourth ventricular solid schwannoma in the literature.[4] The present case is unique in the respect that it resembled the description of a classical Dandy-Walker cyst. The implications in the management strategy of such a case are discussed.

A 15 year old adolescent male, without stigmata of von-Recklinghausen's disease complained of bifrontal headaches and intermittent episodic vomiting for 6 months and visual obscuration for 2 months. On examination his higher functions were normal. Both fundi showed papilloedema. Rest of the neurological examination was normal. Computerised tomography (CT) scan showed a large and nearly rounded hypodense lesion of the cerebellar vermis with no enhancement after contrast [Figure 1]. The lower fourth ventricle was compressed forward and the posterior fossa cisterns were obliterated. The part of the fourth ventricle superior to the cyst, third and lateral ventricles were moderately dilated. The cyst was seen to extend through the foramen magnum into the upper cervical spine. As the ventriculomegaly was only moderate and the cyst was disproportionately large it was decided to drain the cyst. A midline suboccipital craniectomy was done to expose the cerebellar hemisphere and the cisterna magna. On opening of the arachnoid wall of the cisterna magna, another cyst lying within it could be clearly seen. The wall of this cyst was relatively thick and opaque. Pale xanthochromic clear fluid was aspirated from it. The cyst wall was then gently dissected off the cerebellar surface and completely excised. The floor of the fourth ventricle formed the anterior wall and compressed cerebellar tissue, the anterolateral wall of the cyst. These could be seen clearly after the cyst was excised. The cyst was found to be free from cranial nerves or blood vessels. A silastic tube was cannulated through the enlarged aqueduct into the third ventricle and the other end of the tube was placed into the cisterna magna to design an internal shunt. Histopathological examination showed structure of a tumour consisting of spindle shaped cells with elongated nuclei arranged in compact interlacing fascicles. There was a suggestion of nuclear palisading. These features clearly favoured a histological diagnosis of a schwannoma [Figure 2]. Immunostaining could not be performed due to lack of facilities. Repeat CT scan done two weeks after the surgery showed resolution of the cerebellar mass [Figure 3]. At one year follow-up the patient was asymptomatic.

The most common site for intracranial schwannomas is the vestibular portion of the eighth nerve. Although uncommon, these tumours have been reported to occur on every cranial nerve except the optic nerve.[4] Intraparenchymatous schwannomas have also been reported, though rarely.[3] Goebel et al[5] reported a case of an extra-axial schwannoma, not connected to any cranial nerve but attached to the dura in the region of tuberculum sellae and simulated a meningioma. In the present case the cyst appeared to be intracerebellar on preoperative CT scan. However, during the operation it was clearly extra-parenchymal. Magnetic resonance imaging (MRI) could have clarified the location and extensions of the lesion, but was unfortunately not carried out in this patient. The tumour did not have any attachment to the cranial nerves which were in the vicinity. It was also clearly free of the dura and the brain around it. The absence of cranial nerve and brainstem dysfunction, both prior to and after surgery, corroborated this impression. The occurrence of a nerve sheath tumour in such a location raises the important question of its histogenesis. Various theories have been proposed for the origin of intraparenchymal schwannomas. Redekop[4] supported the theory of distorted embryogenesis, while Riggs and Clary[6] postulated that these tumours could arise from proliferation of schwann cells in perivascular plexuses. Russell and Rubenstein suggested that conversion of pial cells to schwann cells was the possible mode of histogenesis.[7] Prakash et al suggested misplaced myelinated nerve fibres as site of origin,[8] while Ramamurthi[9] suggested displaced neural crest cells in the developing nervous system to be the origin of these tumours. Differentiation from multipotent mesenchymal cells has also been suggested.[10] Some cases associated with neurofibromatosis have been reported.[7] We believe that the origin of the tumour in the present case is most easily explained by the theory of distorted extra-cerebral ectopic neural crest cell migration with subsequent neoplastic transformation.[7] The other possibilities being the origin from schwann cells in the perivascular plexus[5] or misplaced myelinated nerve fibres not attached to any of the cranial nerves.[8] The benign nature of this tumour indicates the need for radical excision. Neuroradiological tests are non-specific. Even with CT and MRI, a preoperative diagnosis is impossible. Histopathologically these tumours need to be differentiated from microcystic meningiomas (with prominent nuclear palisading) and pilocytic astrocytomas which previously were called `central schwannomas'.[11] Electron microscopy and immunohistochemistry help in the differentiation.[7] Long term outcome after excision is generally good.[12]